Settings like velocity are grayed out for particles
Hair particles can either be dynamically generated by the particle system or can be cut and placed using the Particle edit. You can't have both.

If you have a close look there aren't that many values that get disabled after you edit particles, and they are all related to the initial generation, like the number of hairs and length. By using particle edit you are saying I want to control the placement of hair particles so don't replace my work with generated hair.